The present invention relates, in general, to thin film electroluminescent display panels, and more particularly to a self-scanning circuit and method of operating such panels which permits rapid entry of picture information for subsequent display.
Electroluminescent devices have been well known for many years, and their potential uses were early recognized. Typical of such devices is that shown in U.S. Pat. No. 2,900,574 to Kazan, which describes the operation of an electroluminescent system for producing a moving spot of light. As therein explained, many phosphor materials may be caused to emit visible radiation by subjecting them to alternating current voltages of sufficient magnitude, applied across the phosphor. Bursts of electroluminescence will occur for each polarity change induced by the AC voltage; accordingly, such a voltage is used to produce a seemingly constant electroluminescence by applying the voltage at a frequency which is shorter than the retentivity of the human eye. Such electroluminescence results from a redistribution of electrons in the crystal structure of the electroluminescent material and the consequent emission of light from that material.
Many attempts have been made to use this phenomenon in the production of a large image utilizing, in a panel or screen, a plurality of discrete electroluminescent pixels, or picture elements, selectively energizable to provide a pattern of dark and light elements to provide the desired image. However, difficulties are encountered in selecting individual pixels in a large matrix of electroluminescent devices, for complex circuitry is generally required, and the application of these devices has, therefore, been limited. Another limiting factor in the production of an image is the relatively short decay time of an electroluminescent device phosphor. Although a short decay time is helpful in the case of a rapidly changing display, if an image is to remain visible for an appreciable time, it is necessary to reestablish the entire image on each cycle of the AC applied voltage to insure that the pattern of illumination is duplicated and that the image is thus retained.
Typically, the various elements in an electroluminescent panel are connected in an arrangement of the type shown in U.S. Pat. No. 3,054,929 to Livingston, U.S. Pat. No. 2,925,532 to Larach, or U.S. Pat. No. 3,627,924 to Flemming. All of these provide display panels which utilize a matrix of electroluminescent devices connected in a crossed grid array, having rows and columns of excitation conductors with electroluminescent devices connected therebetween at the grid intersections. Energization of a selected row and a selected column conductor applies a voltage across the single electroluminescent device located at the intersection of the row and column conductors. This allows the unique selection of each electroluminescent pixel in the matrix. To provide a complete display, the row conductors are scanned and at the energization of each row, the desired column conductors are selected to energize the electroluminescent devices in that row which correspond to the selected column conductors. The rows are energized sequentially and selected columns are energized during the selection of each row. When this is done very rapidly, the electroluminescent devices which are energized provide the desired pattern of illumination. Because each phosphor has a finite decay time, the image will quickly fade away if the same sequence of scanning the rows and selecting the columns is not immediately repeated, and by carrying out such repetition at a sufficiently high rate, the device will provide an apparent continuous image. The patents listed above provide variations on this general approach, the Larach patent showing a polychromic display by making adjacent rows one of three different colors, the Livingston patent showing the use of a beam switching tube for a crossed grid matrix, and the Flemming patent showing the use of electroluminescent devices for a TV display.
The cross grid or matrix systems of the prior art require extensive, complex, control circuitry. For example, if a matrix has M columns and N rows, then M+N control switching circuits are required to select and illuminate the individual electroluminescent devices in the matrix. For a large display panel, the number of circuits and the complexity of the control presents serious cost and reliability problems. Furthermore, to sustain a given picture, repetitive selection of each element in the display is required, further complicating the control problems.